Stopping corrosion

When heat stable salt buildup becomes a problem a variety of options may manage it. These include partial or total solution replacement, heat stable salt removal, or adding caustic to neutralize the heat stable salts. Many operators choose caustic addition because it is perceived to be a more economical way to stop corrosion and subsequent foaming and loss problems. 

Cathodic protection is unique amongst all the methods of corrosion control in that if required it is able to stop corrosion completely, but it remains within the choice of the operator to accept a lesser, but quantifiable, level of protection. Manifestly, it is an important and versatile technique. 

There are complications however. The cathodic potential effectively stops corrosion on the metal substrate but it also contributes to early debonding of adherends, the development of leak paths under seals, and the blistering and peeling of coatings. Degradation of protective coatings is a basic life-limiting problem for underwater equipment exposed to a cathodic potential. 

Corrosion of the cell body occurs when the electrolytic current has stopped. Corrosion products, such as iron(III) fluoride, arc a cause of polarization. The cathodic protection method is applied to prevent this corrosion problem. Iron(III) fluoride can be eliminated from the electrolyte as a precipitate on addition of sodium fluoride. 

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

Since both the cathodic and anodic steps must take place for corrosion to occur, prevention of either one will stop corrosion. The most obvious strategy is to stop both processes by coating the object with a paint or other protective coating. Even if this is done, there are likely to be places where the coating is broken or does not penetrate, particularly if there are holes or screw threads. 

The methods used to inhibit and effectively stop corrosion can be divided into two large groups depending on the corroding situation. If the body to be protected is in an infinitely large solution, typically the sea, then the methods have to rely upon what can be done electrochemically to the metal itself. Usually, as in the widely practiced cathodic protection, a circuit is fixed up in which the metal of the object to be protected is moved away from the corrosion potential in the cathodic direction (or electronation), thus reducing the anodic (or deelectronation) dissolution velocity, and hence the corrosion. 

Surprisingly, it is better to coat steel with another metal that does corrode. Trash cans, for example, are made of zinc-coated steel. This coating does not stop corrosion. But the zinc corrodes first, making the steel underneath last much longer than it would without the zinc layer. 

Determine the pressure of hydrogen required to stop corrosion of iron immersed in a deaerated 0.1 m FeCl2 solution at pH = 3. Assume YFe2+ = 1.0. (Ref 11)... 

Figure 18.3 Principles of repair to stop corrosion of reinforcement, according to Rilem 124-SRC [1]...

This technique requires the permanent application of a small direct current to protect the steel. It can also lead to repassivation of the reinforcement if it lowers the steel potential below the repassivation potential (Section 7.3). Provided it is applied properly, cathodic protection is able to stop corrosion for any level of chloride con-... 

Corrosion inhibitors. Corrosion inhibitors may in principle improve the protection of the steel reinforcement. Mixed-in inhibitors added to repair mortar may increase the chloride threshold or delay the chloride penetration in the repair material. Migrating inhibitors appHed on the surface of the original concrete are intended to reduce or stop corrosion and thus make concrete removal and its replacement unnecessary. The role of inhibitors and their effectiveness are discussed in Chapter 13. It is, however, useful to remember that if a corrosion inhibitor is used, the concentration that is needed in the vicinity of the reinforcement should be specified and suitable means for demonstrating that such conditions are actually achieved and maintained for an adequately long time should be proposed [11-... 

It is impossible to remove all the chloride from concrete by the electrochemical method. But the level of chloride in contact with the steel is reduced by 45-95%. Field data have shown that the removal of chloride by electrochemical technique results in stopping corrosion for 8 years. It is predicted by FHWA that the electrochemical method of removal of chloride will extend the life of bridges by as much as 20 years (31). About 372,000 nP (4,000,000 ft ) concrete has been treated by this method. 

Nickel is deposited at a current density of 75 A/m. Calculate the limiting current if the reduction occurs at a concentration overpotential of—150 mV. Calculate the corrosion potential, corrosion current, and protection current needed to stop corrosion for cadmium in a corrosive deaerated medium. Additional information ... 

Kuhn [19] ilrst postulated in 1933 that the potential needed to stop corrosion is probably in the neighborhood of —0.85 V vs. CU/CUSO4. The results obtained from extensive studies on cathodic protection [20-27] helped National Association of Corrosion Engineers (NACE) to establish criteria for cathodic protection [28]. NACE RP-01-69 specifies A negative (cathodic) potential of at least 850 mV vs. Cu/CUSO4 should be appfied to protect the structure [28,29], However, in the presence of sulfides, bacteria, elevated temperatures, acid environments, and dissimilar metals, the criteria of—850 mV may not be sufficient [5,30—33]. According to NACE, one should also account for the IR drop at the metal-soil interface, which is included in most practical measurements and is an uncertain value depending on the electrolyte (soil) resistance. 

The essential methods for stopping corrosion are changing the material, changing the environment, and protecting the material. 

While these may be an oversimplilication, they tell the maintenance engineer, with the assistance of the materials engineer, the steps to carry out in stopping corrosion. 

How can hazard reduction occur by stopping corrosion in design ... 

---